Raman spectroscopy, as well as variants such as resonance Raman and surface-enhanced Raman spectroscopy, is attractive for remote sensing applications due to its selectivity. The vibrational information inherent in a Raman spectrum can potentially be used to discriminate among a large number of analytes, allowing molecules to be identified and concentrations to be determined. Efforts have been made to develop remote sensing techniques for groundwater contaminants using remote Raman spectroscopy over optical fibers. Of particular interest are resonance Raman (RR) and surface-enhanced Raman (SER) spectroscopies because these techniques make it possible to measure certain environmental contaminants at very low levels.
Raman measurements over optical fibers are more difficult than fluorescence measurements. First, Raman signal intensities are generally much weaker than fluorescence signal intensities. Furthermore, the wavelengths of the Raman bands are usually much closer to the laser wavelength than are fluorescence bands and thus require very good background rejection in the spectrometer.
The possibility of making Raman measurements with optical fibers has been shown by others. However, there have been no published reports of Raman measurements in the "signature" region using very long optical fibers, and the technique has not been widely employed to date.
A major obstacle in the successful exploitation of Raman spectroscopy with long optical fibers is interference from the large Raman background emission of the fiber itself. This background emission is structured, making effective subtraction of it difficult and the detection of weak signals with single-fiber probes impossible with all but the shortest fibers. As a result of this difficulty, multiple-fiber optrodes in which the functions of excitation and collection are performed by different fibers were developed. Multiple-fiber optrodes have decreased sensitivity compared with that for an ideal single fiber, because overlap of the excitation and collection volumes is less than it is for a single fiber. The use of additional collection fibers increases the sensitivity, but long optical-fiber bundles are prohibitively expensive, and efficient coupling of a fiber bundle into a spectrometer slit may present some difficulties. Also, fiber bundles do not eliminate the fiber background for highly scattering, samples and may have limited use for many practical applications.
What is needed in the art is a device or apparatus which enables detection of Raman spectra at long distances, and with a minimum of background noise.
This invention is concerned with such apparatus and associated methods.